Tuning In the Planet's Hum

Water often makes people hum if they're standing in a shower. Surging water makes Earth hum as well, but in a rather different way. A new study shows that storm-driven waves pulsate against the ocean floor and set the whole planet ringing with faint vibrations, explaining one of seismology's most intriguing puzzles.

Large earthquakes make Earth resonate like a gong struck by a hammer. Seismic recorders pick up these "free oscillations," which make the planet throb ever so slightly once every few minutes. Six years ago, a Japanese team found that Earth oscillates all the time, even without triggers from quakes (ScienceNOW, 26 March 1998). Some researchers suspected that pressure changes in the air push hard enough on Earth's surface to set off the minuscule seismic waves. Others thought ocean waves are more likely sources, perhaps from their cumulative pounding of the shore.

Now it appears the ocean is indeed what sets the Earth humming, but the vibrations are set off by large stormy waves in the open sea, not breakers on the beach. Graduate student Junkee Rhie and seismologist Barbara Romanowicz of the University of California (UC), Berkeley, used instruments in California and Japan tuned to the longest of the seismic ripples that traverse Earth's surface. The networks monitored the planet's free oscillations on days when few earthquakes happened. The detectors were spread out across hundreds of kilometers, allowing Rhie and Romanowicz to trace the faint signals back to their sources.

The hums arose in two vast expanses of ocean: the northern Pacific Ocean during the winter and the southern oceans around Antarctica during the summer (winter in the Southern Hemisphere). According to Romanowicz, that alternating pattern matches satellite images of the biggest storm-generated waves. "Winter winds create big waves that make the ocean slosh around against the sea floor, exerting pressure on the solid Earth," she says. The team's analysis appears in the 30 September issue of Nature.

The results support a scenario put forward last year by seismologist Toshiro Tanimoto of UC Santa Barbara, in which ocean waves with the proper timing--peak to peak periods of about 3 to 5 minutes--excite the planet's hum by creating oscillations that reach all the way to the bottom. "It's a nice piece of work," Tanimoto comments. Still, he notes, some other researchers may question whether the arrays of seismic instruments can reliably track the directions of the barely perceptible vibrations.